The most important parameter of the chemical enrichment is expected to be the
star formation history (SFH). Since massive stars dominate the production
of most elements, the metal
production rate will to a first approximation be directly proportional to the
star formation rate. Likewise, in a simple chemical evolution picture, the
metallicity is a function of the fraction of gas turned into stars.

Star formation (SF) seems to occur in different modes: one being relatively
undramatic with continuous star formation at a regular pace while
in ``bursting'' galaxies, star formation may be dominated by a small
number of
short intense bursts of star formation separated by extremely long intervals
of time. While the latter scenario is mainly observed in blue compact
galaxies it is believed to occur also in dwarf and giant elliptical
galaxies. An intermediate
picture is that of ``gasping'' SF, characterised by extended star formation
episodes separated by moderately long periods of less active SF, which
probably is the most realistic picture for many dwarf irregular
galaxies. While a starburst produces a lot of metals, the
supernovae and stellar winds may eject gas into the intergalactic
medium on short time scales. If, on the other hand,
the SF is continuous, the energetic feedback from dying stars will have
less influence on the ISM.
Different scenarii for SF regulation have been implemented with stochastic
self-propagating star formation, self regulated star formation and also
a gas density threshold. Moreover interactions, mergers and stripping
may play an important rôle in regulating the SF.

Discrete star-bursting behaviour in dwarf galaxies may strongly affect
the abundance
ratio of elements such as N, Fe and C which partly come from longer lived
intermediate mass stars as compared to O
(Garnett 1990,
Gilmore and Wyse 1991,
Richer and McCall
1995).
In particular, this could explain the tendency of C/O and Fe/O to
increase with O/H
especially at low O/H, but this awaits more accurate measurements of
these ratios in extremely metal-poor galaxies. Moreover, since the net
yield for many elements appears to be metallicity dependent
(Maeder 1992),
element production
by subsequent generations of stars will depend on how fast the ISM is enriched
and hence the star formation history. In addition, variations in element ratios
could affect nucleosynthesis and thus the net yields for a given stellar
generation.

A last note concerns ``starbursts'': This notion is frequently used in
the literature to describe regions/galaxies with varying degree of active star
formation. A proper definition of a starburst is that it involves
an unsustainably high star formation rate (SFR) in terms of the gas
consumption timescale or the timescale to
build up the observed stellar mass (i.e., the time averaged SFR is much
lower than the present). Many galaxies have SFRs fluctuating with
time, but this does not necessarily imply that the SFR is unsustainable over a
Hubble time.